In automation technology, especially in process and factory automation technology, field devices are often applied, which serve for registering and/or influencing process variables. Serving for registering process variables are sensors, such as, for example, fill level measuring devices, flow measuring devices, pressure- and temperature measuring devices, pH-redox potential measuring devices, conductivity measuring devices, etc., which register the corresponding process variables, fill level, flow, pressure, temperature, pH-value, and conductivity, respectively. Serving for influencing process variables are actuators, such as, for example, valves or pumps, via which the flow of a liquid in a pipeline section, respectively the fill level in a container, can be changed. Referred to as field devices are, in principle, all devices, which are applied near to the process and which deliver, or process, process relevant information. Understood to be field devices in connection with the invention are also remote I/Os, radio adapters, respectively, in general, devices, which are arranged at the field level and communicate with one another via a data bus. Often, one includes, in this connection, also network components. A large number of such field devices, or network components, are produced and sold by the firm, Endress+Hauser.
However, the invention is not limited to field devices in the above described sense. In general, the invention is applicable for all technical devices, in the case of which electromagnetic compatibility (EMC) plays a role. Electromagnetic compatibility designates the desired condition that devices do not interfere with one another due to electrical or electromagnetic effects. Expressed in other words: In general, the invention is applicable in the case of all devices, in the case of which components (housing, adapters, etc.) of a non-conductive material must be electrically contacted via a metallized screw thread.
Field devices usually have a housing for accommodating an electronics unit and a sensor unit, The housing is pot shaped and is often manufactured of a non-conductive material. Suitable materials include synthetic materials, e.g. plastics, or composite materials. Further provided on the housing is at least one line guide passageway having a screw thread. Electrical lines are led out of the housing through the line guide passageway. In order to prevent electrostatic charging of the housing, at least the inner surfaces of the housing and the line guide passageway with screw thread are provided with a conductive coating.
In the case of the metallizing, respectively in the case of the coating with a conductive material, the particles do not deposit uniformly and continuously in the threads of a passageway. As a result of this, there arises along the thread of the passageway a helically shaped path, which significantly lengthens the distance, which the electrical charges travel in the region of the passageway. The lengthening of the distance leads automatically to an increase of the resistance between two measurement points, of which one measuring point is located within the housing and the other measuring point is located outside of the housing. This is naturally disadvantageous for draining away undesired charges.
For a long time, this problem has been solved by grounding the field device via a supplemental part, e.g. a bridging strut or a metal contact strip. The corresponding supplemental parts bring the following disadvantages: logistical effort, caring for the material, assembly effort, sealing problems, additional costs- and time consumed, poor appearance, risk that contaminations collect, risk of injury, etc.